Association of weekend catch-up sleep ratio and subjective sleep quality with depressive symptoms and suicidal ideation among Korean adolescents

Circadian misalignment caused by differences in sleep duration between weekends and weekdays may be associated with adolescent mental health and sleep quality may be able to compensate for this problem. This study aimed to investigate the association between weekend catch-up sleep (CUS) ratio and sleep quality with depressive symptoms and suicidal ideation among South Korean adolescents. We used data from the Korea Youth Risk Behavior Web-based Survey 2015–2019 involving 270,619 adolescents. The weekend CUS ratio was calculated by dividing the average weekend sleep duration by the average weekday sleep duration (< 1.00, 1.00 ≤ CUS < 1.50, or ≥ 1.50). Subjective sleep quality was categorized as poor, moderate, or good. Multiple logistic regression analyses were performed. A weekend CUS ratio of < 1.00 and poor sleep quality was significantly associated with mental health. Absolutely short sleep duration (CUS < 1.00 and weekday sleep duration < 5 h) was most associated with depressive symptoms and suicidal ideation. Furthermore, adolescents with a CUS ratio of ≥ 1.50 showed increased odds of depressive symptoms despite having good sleep quality. Appropriate weekend CUS may benefit adolescents’ mental health. When investigating the relationship between adolescents’ sleep and mental health, a weekend CUS ratio should be considered in addition to sleep quality and duration.


Methods
Study population and data. We used data obtained from the Korea Youth Risk Behavior Web-based Survey (KYRBS) during 2015-2019 for this cross-sectional study, which included a nationally representative sample of South Korean adolescents attending middle and high school (grades 7-12, age [12][13][14][15][16][17][18]. All participants voluntarily log in using their certificate number and check the online informed consent themselves 26 . The KYRBS is a kind of Korean government-approved statistics (Approval number: 117058), and an anonymous self-administered structured questionnaire that does not involve any intervention on adolescents. Thus, ethics approval and informed consent of parents or legal guardians for the KYRBS was waived by the Korea Disease Control and Prevention Agency Institutional Review Board in accordance with the Bioethics and Safety Act, 2015 27 . The KYRBS complied with the Declaration of Helsinki's ethical standards and used a complex research design, which included multistage sampling, stratification, and clustering. The KYRBS was conducted through an online survey system, and the questionnaire comprised approximately 120 items in 15 sections. Respondents could not move on to the next section unless all the questions were answered in the current section, although responses with logical errors and responses that were outliers were treated as missing data. A total of 270,619 participants (male, 136,316 [50.4%]; female, 134,303 [49.6%]) were included in the study, excluding those who were treated as missing data due to logical errors or outliers in sleeping and waking times.
Variables. The main dependent variables were depressive symptoms and suicidal ideation. Depressive symptoms were assessed by the question "In the last 12 months, have you felt sad or hopeless enough to stop your daily activities for 2 weeks?. " Suicidal ideation was assessed by the question "Have you seriously considered suicide in the past 12 months?. " These variables were both dichotomized into a "yes" or "no" response.
The main independent variables were the weekend CUS ratio and subjective sleep quality. First, to determine the weekend CUS, the average sleep duration on weekdays and weekends was calculated using the question "During the past 7 days, what time did you usually go to bed and wake up?" In response to this question, the participants recorded the time they went to bed and the time they woke up on weekdays and weekends, respectively. Based on the recorded times, the average sleep time on weekdays and weekends was calculated.
The weekend CUS ratio was calculated by dividing the average weekend sleep duration by the average weekday sleep duration, according to the responses of the survey.
According to this formula, the weekend CUS ratio was classified as "CUS < 1.00," "1.00 ≤ CUS < 1.50," and "CUS ≥ 1.50. " For example, a teenager who slept 6 h during the weekday and 3 more hours on the weekend would be in the "CUS ≥ 1.50 (6 × 1.5 = 9 h)" group. However, a teenager who slept 7 h during the weekday and 3 more hours on the weekend would be included in the group with a "1.00 (7 h) ≤ CUS < 1.50 (10.5 h)".
The weekend CUS ratio of 1.00 was used to confirm the association between shorter sleep duration on weekends than on weekdays and mental health of adolescents. The upper cut point for CUS of 1.50 means that the average sleep duration on weekends is 1.50 times higher compared to the average sleep duration on weekdays.
Weekend catch up sleep ratio = Average weekend sleep duration (hours) Average weekday sleep duration (hours) The upper cut point for CUS was selected using the following rationale: First, the appropriate amount of sleep for adolescents is 8-10 h 13 , which is 1.50 times greater than the average sleep duration for South Korean adolescents (i.e., 6-7 h 15 ) on weekdays. Next, considering the previous studies that defined extremes when the absolute difference in sleep duration between weekdays and weekends was more than 2 h 21 , and that students in South Korea additionally slept for about 3 h more on weekends 14 , weekend sleep duration within 1.50 times of weekday sleep duration was set as normal. Subjective sleep quality was measured by asking the participants whether their sleep was sufficient to recover from fatigue during the last 7 days. The five possible responses were: very good, good, moderate, poor, and very poor. Finally, we grouped subjective sleep quality into three categories: "good (very good and good), " "moderate, " or "poor (poor and very poor)".

Statistical analyses.
The analyses were performed separately by sex, considering gender differences in depressive symptoms and suicidal behaviors 28 . We performed chi-squared tests to assess differences between frequencies and proportions of categorical variables. To analyze the association of the weekend CUS ratio and subjective sleep quality with depressive symptoms and suicidal ideation, we used a multiple logistic regression analysis after adjusting for covariates. For evaluating the interaction between the weekend CUS ratio and weekday sleep duration and the interaction between the weekend CUS ratio and subjective sleep quality, additional multiple logistic regression analyses were performed. Finally, we performed sub-group analyses for the association of the weekend CUS ratio with depressive symptoms and suicidal ideation, stratified by residence type and perception of stress, as two covariates are closely related to sleep and mental health in adolescents [29][30][31] . All statistical analyses were performed using SAS version 9.4 (SAS Institute, Cary, NC, USA), and a weighted logistic regression procedure was conducted to account for the complex and stratified sampling design of the KYRBS. P-values < 0.05 were considered statistically significant.

Discussion
In this study, both male and female adolescents whose weekend CUS ratio was 1.00-1.49 had fewer depressive symptoms and less suicidal ideation. Further, fewer depressive symptoms and less suicidal ideation were observed in participants with better subjective sleep quality. This demonstrates that the imbalance between weekday and weekend sleep was associated with experiencing depressive symptoms and suicidal ideation in adolescents.
To cope with weekday sleep debt, weekend CUS is associated with a lower prevalence of obesity, hypertension, anxiety, and depression 32,33 . Moreover, a recent study of South Korean high school students reported that adolescents' weekend CUS duration of > 2 h is associated with a lower risk of depression 19 . However, no studies have confirmed whether longer weekend CUS is associated with depression and suicidal ideation, which are important mental health conditions in adolescents, although a long sleep duration may have adverse health effects 34,35 . To the best of our knowledge, this is the first study to confirm, using the weekend CUS ratio, that an imbalance between weekday and weekend hours is associated with increased odds of depressive symptoms and suicidal ideation in adolescents.
In our study, adolescents with a weekday sleep duration of < 5 h and weekend CUS ratio of < 1.00 showed particularly high levels of depressive symptoms and suicidal ideation. It is estimated that such results are due to an absolute lack of sleep. It has been reported that irritation, anxiety, and feelings of worthlessness are found in adolescents with a short sleep duration 36 . Shorter-duration sleepers are also less motivated and more likely to have trouble focusing 36 . Our findings support previous research showing that short sleep duration, including during weekends, can be associated with depressive symptoms or suicidal feeling in adolescents 18,36,37 .
When the quality of sleep and the CUS ratio were considered concurrently in adolescents with good sleep quality, no association between depressive symptoms and suicidal ideation was observed even if the CUS ratio was < 1.00. However, contrary to our expectation, if adolescents with good subjective sleep quality have a ≥ 1.50 times longer sleep duration on weekends than on weekdays, the odds of having depressive symptoms increases. One of the causes of reward-related problems, such as depression, may be circadian misalignment, which implies greater weekend-weekday advances in mid-sleep 38 . Circadian misalignment is associated with a decrease in the medial prefrontal cortex and striatal reactivity to reward, which may reflect a decreased reward sensitivity 38 . Sleep-wake homeostatic dynamics might also have influenced these results 39,40 . Moreover, while insufficient sleep is obviously associated with negative health effects, habitual excessive sleep can also increase mortality risks. In contrast, long sleep may lead to sleep fragmentation, fatigue, depression, photoperiodic abnormalities, or lack of challenge, which establishes a mortality link 41 . These results also suggest that the circadian misalignment can be compensated to some extent by the subjectively perceived quality of sleep, but if the degree of misalignment is severe, the compensatory effect disappears. Further biological studies are needed to clarify the causes of long sleep duration or how circadian misalignment adversely affects the mental health of adolescents.
We performed sub-group analyses considering the effect of family environment on sleep disorders 29 , the relationship between family environment and mental health 30 , and the relationship between perceived stress and sleep duration with metal health 31 . In the stratified analysis, the trend of association was significant, depending on residence type and perception of stress. When the weekend CUS ratio was < 1.00, male and female adolescents  www.nature.com/scientificreports/ living with relatives or with low perceived stress levels were more likely to experience depressive symptoms and suicidal ideation. Previous studies reported that adolescents living apart from one or both biological parents showed association with suicidality 30,42 . However, to the best of our knowledge, no study has investigated the association between mental health and sleep duration, especially weekend CUS, among adolescents living with relatives. Our results imply that both the living environment and sleep-related factors should be considered to safeguard the mental health of adolescents who do not live with their parents. Furthermore, since the absolute lack or imbalance between weekday and weekend sleep durations had a greater effect on adolescents with low stress levels, an approach that considers the weekend CUS ratio is necessary in diagnosing and treating adolescent mental health disorders, in addition to the stress level.
Although the absolute amount of sleep is supposed to have more influence on the level of depressive symptoms and suicidal ideation than the weekend CUS ratio, the balance of sleep duration between weekdays and weekends also showed a meaningful association with the level of depressive symptoms and suicidal ideation after adjusting for average weekday and weekend sleep durations. In fact, previous findings also indicated that adolescents experience longer sleep duration on weekends than on weekdays 14,20 . This is the consequence of delayed circadian phase and compensation of the sleep debt from the weekdays' lack of sleep in the form of weekend CUS 43 . Hence, to protect adolescents from depressive symptoms and suicidal ideation, we should identify ways to achieve balance of sleep duration between weekdays and weekends, as well as a sufficient amount of sleep. With sufficient amounts of sleep, good balance of sleep duration between weekdays and weekends and subjectively good quality of sleep can lower the level of adolescents' depressive symptoms and suicidal ideation.
Therefore, we should recognize the importance of sleep hygiene education and sleep intervention to prevent depression or suicide in adolescents. Furthermore, personal and institutional efforts must be undertaken, which should include the provision of social systems and campaigns for sufficient sleep with good balance and quality 44 . Additionally, personal efforts are needed to maintain enough sleep with good balance and quality. Further research is needed to investigate the psychosocial and neurobiological mechanisms to explain the association of inadequate balance of sleep duration between weekdays and weekends with adolescents' mental health.   www.nature.com/scientificreports/ Previous studies using the KYRBS suggested that sleep duration and sleeping time are associated with mental health concerns such as suicidal behaviors 27,[45][46][47][48] . That is, a sleep duration of less than 4 h was associated with suicidal ideation [45][46][47] , and a prolonged time to fall asleep and wake up was also associated with suicidal ideation and suicidal ideation 27,48 . Our research is meaningful because we additionally analyzed the balance of sleep duration between weekdays and weekends using a "weekend CUS ratio. " Overall, our findings serve as a warning regarding the risk of depression or suicidal behaviors in adolescents with an imbalance of sleep duration between weekdays and weekends, and this is an important hypothesis to be confirmed in future neurobiological studies. Furthermore, because this cross-sectional study used multiyear national survey data (2015-2019) with 270,619 respondents based on random cluster sampling, our results were sufficiently representative of South Korean adolescents 30,49 . Additionally, we considered many covariates to increase the credibility and accuracy of our study.
Despite these strengths, our study had some limitations. First, the study's cross-sectional design did not allow us to clearly identify the direction of the association of weekend CUS and subjective sleep quality with depressive symptoms and suicidal ideation. Second, the results of this study were based on anonymous selfreported data. Thus, the weekend CUS ratio and subjective sleep quality might have been underestimated or overestimated, and some survey questions might be subject to recall bias 30 . Third, as the dependent variables, depressive symptoms and suicidal ideation, consist of single-item questions like other population-based studies such as YRBS 50,51 , it may have caused measurement problems and misclassification 52 . However, according to the results of one study, the reliability of the questionnaire for depressive symptoms and suicidal ideation in YRBS, was "moderate" or higher 53 . Since KYRBS uses a questionnaire similar to YRBS, reliability can be guaranteed to a certain extent, but additional studies are needed. Moreover, the previous study that shorter of longer sleep duration was associated with the depressive symptoms measured by Patient Health Questionnaire helps to give meaning to this study that suggested the relationship between the CUS ratio, one of the characteristics of sleep, and depressive symptoms [54][55][56] . Finally, despite our efforts to control for confounding factors, not all covariates affecting depressive symptoms and suicidal ideation were considered. Therefore, the possibility of residual confounding cannot be eliminated.
In conclusion, our study demonstrated that both male and female adolescents with a weekend CUS ratio of < 1.00 were significantly more likely to have depressive symptoms and suicidal ideation. Furthermore, poor sleep quality was associated with increased odds of having depressive symptoms and suicidal ideation. Male and female adolescents with a weekday sleep duration of < 5 h and weekend CUS ratio of < 1.00 showed the highest increased odds of depressive symptoms and suicidal ideation. In contrast, adolescents with good subjective sleep quality and a weekend CUS ratio of ≥ 1.50 showed a higher prevalence of depressive symptoms. Further study is needed to examine the neurobiological mechanisms underlying the association of the balance of sleep duration with adolescents' mental health. At the same time, policy makers need to consider appropriate education and interventions that can balance sleep duration on weekdays and weekends, as well as adequate sleep duration during weekdays, to improve adolescents' mental health.

Data availability
All the KYRBS data used this study are available to the public and can be seen in the KYRBS official website (http:// www. kdca. go. kr/ yhs/).